It has been awhile, but they are filming Series Four (Going with the British terminology today), I have rewatched "The Abominable Bride", and the game is on!
As always ***SPOILER ALERT***
Let's begin with two facts which shall spark our discussion today:
1.) Moriarty is dead
2.) There is no twin
You cannot survive a gunshot to the head, and you cannot fool Sherlock into thinking you have shot yourself in the head. Moriarty is dead.
I may have been a proponent of the Moriarty brother theory before, but I am no longer. As Sherlock says, "It's never a twin."
What caused the apparent return of Moriarty then?
To figure it out Sherlock went back in time (sort of). "The Abominable Bride" is set almost entirely in a world created in Sherlock's dream-like state. Therefore, every part of that story is significant.
Let's explore the good ol' United States. There is some connection to the United States; there has to be. Think back to "The Great Game." Every time Moriarty called Sherlock he heard pips. In Sir Arthur Conan Doyle's short story "The Five Orange Pips", Sherlock Holmes explains that orange pips were used as warnings to members of secret societies, a fact brought up in both "The Great Game" and "The Abominable Bride." Moriarty has some connection to the United States and secret societies. Who else has a connection to the United States. The Woman, for one, was being chased by Americans when she first appeared in Sherlock. She could be involved. Someone else with a possible connection to the United States and more importantly in a position to mess with Sherlock is one Mary Watson. In "His Last Vow", Sherlock deduces that Mary is not English. Her closeness to John and Sherlock gives her ample opportunity to mess with them, but why would she do that?
Moriarty was running a vast criminal network when he died. He had minions. As he told Sherlock, he planned to get "a live-in one." Perhaps, The Woman or Mary is working for Moriarty. He has a criminal network as mirrored in "The Abominable Bride." The network in the Chrismas special was comprised of women. A femme fatale could very likely be at the center of Series Four.